Andreev reflection tunneling through a triangular triple quantum dot system

By using the nonequilibrium Green's function technique, we study the Andreev reflection (AR) in a triangular triple quantum dot system coupled with ferromagnetic and superconducting electrodes. An expression for the AR conductance is derived at zero temperature. It is found that the AR process does not break time-reversal symmetry due to the AR coefficient as an even function of the magnetic flux. Similar to the matching condition of the Fermi velocities, the zero-bias conductance reaches its maximum provided that the matching condition is satisfied. One can observe that the spectrum of the AR conductance presents the resonant peaks, the dips or the Fano-like shape, and the spin polarization of the ferromagnetic lead and the magnetic flux strongly affect the AR process. The dependence of the AR current on the temperature is also discussed. These results provide the ways of modulating the AR process.